The genetic lysosomal storage disease mucopolysaccharidosis IIIB (MPS IIIB; also called Sanfilippo B syndrome) causes progressive intellectual impairment and behavioral problems beginning in early childhood, culminating in neurological devastation and death, usually by the third decade of life. Enzyme replacement therapy (ERT) has been successfully developed and clinically deployed for other mucopolysaccharidoses, notably MPS I, II, and VI. MPS IIIB is due to deficiency of a soluble lysosomal hydrolase as well, and theoretically should be treatable with ERT. However, the barrier to developing ERTF for MPS IIIB is the fact that the enzyme that is deficient in MPS IIIB (?-N-acetylglucosaminidase, or NAGLU) naturally has sufficient mannose 6-phosphate moieties to enable efficient cellular uptake via the mannose 6-phosphate receptor pathway, but recombinantly-produced NAGLU enters cells inefficiently because it contains little or no mannose 6-phosphorylation. To overcome this hurdle, we developed a fusion protein of insulin-like growth factor 2 (IGF2) and NAGLU (rhNAGLU-IGF2). IGF2 is a natural ligand of the mannose 6-phosphate receptor (a scavenger receptor that is also called IGF2 receptor), and thus provides a way for NAGLU to exploit this receptor for cellular uptake and lysosomal targeting in the absence of mannose 6-phosphate residues. We propose that the IGF2 peptide fused to NAGLU will increase the therapeutic potential of NAGLU by enabling the enzyme to efficiently enter cells and traffic to the lysosomes. To develop this further, we propose preclinicl studies to determine whether intraventricular rhNAGLU-IGF2 safely and effectively reduces lysosomal storage in MPS IIIB (Naglu-/-) mice (Aim 1) and to determine the brain distribution of intraventricular rhNAGLU-IGF2 (Aim 2). To achieve these objectives, we will deliver rhNAGLU, the rhNAGLU-IGF2 fusion, or vehicle control intraventricularly to MPS IIIB mice. We will then perform detailed pathologic, immunohistochemical, confocal microscopic, and biochemical functional analyses, as well as distribution and penetration studies as a function of dose and time following delivery. Our preliminary data show that rhNAGLU- IGF2 enters MPS IIIB fibroblasts far more efficiency than rhNAGLU. The clinical relevance of this work is supported by the following: a) recombinant ERT is approved and marketed for several other MPS disorders and lysosomal storage diseases; b) clinical trials of intrathecally-delivered enzyme replacement therapy are ongoing for three other MPS types, but not MPS IIIB; and c) a clinical trial of an IGF2-tagged lysosomal enzyme (acid alpha-glucosidase) has recently begun. All these not only establish precedence for translational relevance, but will guide development of our proposed application with rhNAGLU-IGF2. Thus, we are optimistic that rhNAGLU-IGF2 may be successfully developed to treat a devastating, fatal disease for which no treatment exists, and furthermore, that this approach may ultimately be adapted to other genetically-based lysosomal storage diseases as well.